The invention pertains to machine vision. It has application, by way of example, in alignment of wafers during semiconductor wafer fabrication.
Semiconductor chip manufacture is a multistage process in which silicon or other wafers are subjected to a series of processing steps by modular wafer processing units or “systems.” Those units, each of which typically performs one or more dedicated processing steps, are arrayed along the floor of a factory or “fab.” Wafers are transported among and between the units by conveyors, robots and so forth.
Before a wafer can be processed by a processing system, the wafer's position and alignment must be established. This insures that lithography and/or other steps performed by the unit are properly directed, both with respect to features inherent to the wafer (e.g., directionally-oriented crystalline structures) and with respect to processing already (or to be) performed on it by other units.
In most of today's modular wafer fabrication systems, initial alignment of the wafer is performed by a mechanical pre-aligner that spins the wafer and uses a linear camera to find the wafer's alignment indicator (e.g., a notch or “flat” along the perimeter of the round wafer). Once it finds the notch (or flat), the pre-aligner spins a chuck and platform on which the wafer sits in order to orient the wafer to a known angle and, thereby, to ready it for processing within the unit. The pre-aligner may be used repeatedly during processing of a single wafer and/or prior to its transfer to another unit in the fab.
A problem with today's pre-aligners is that they are expensive and slow. They can cost up to $20,000 and typically require up to five seconds to perform an alignment. Since each additional second consumed during pre-alignment can cost the fab thousands of dollars, or more, it is desirable to reduce those times, as well as the cost of the pre-aligners themselves. Another problem is that they can be inaccurate and/or require touching the backside of each wafer. Since today's pre-aligners spin wafers on a mechanical chuck, they can only be as accurate as allowed by the mechanical tolerances of the spinning chuck.
An object of the invention is to provide improved methods and apparatus for machine vision.
A related object is to provide such methods and apparatus as can be applied in semiconductor wafer fabrication.
Still further objects are to provide such methods and apparatus as can be applied in alignment of wafers.
Yet still further objects of the invention are to provide such methods and apparatus as can be used in wafer fabrication systems.
Still yet further objects of the invention are to provide such methods and apparatus as operate quickly.
Yet still yet further objects of the invention are to provide such methods and apparatus as can be implemented at low cost.
Further objects of the invention are to provide such methods and apparatus as improve the accuracy of pre-aligners so as to handle the smaller tolerances of each successive generation of semiconductor manufacturing technology.
Still further objects of the invention are to provide such methods and apparatus as facilitate the use of pre-aligners that avoid touching the backside of wafers that are being processed and that, therefore, improve yield by reducing contamination.
Still yet further objects of the invention are to provide such methods and apparatus as can be used with extant and future wafer fabrication processes and equipment.